Field of the Invention
The present invention relates to a cable television network (CATV network) and a channel selection type radio transmission apparatus and, more particularly, to a cable television network for transmitting video signals to subscribers through cables and a channel selection type radio transmission apparatus which is provided on a cable branch side so as to transmit video signals on radio waves for a distance of about the last 100 m (between the subscriber's house and a terminal of the cable branch).
In a cable television network (CATV network), video signals for several ten channels are transmitted from the center station to a subscriber's house on optical cables or coaxial cables in the form of FDM (frequency division multiplex). In such a cable television network, it is sometimes impossible to install a cable from a cable branch point (tap) to a subscriber's house. For example, when there is a road, railway or a river between a cable branch point and a subscriber's house, it is difficult to install a cable therebetween. When the distance between a cable branch point and a subscriber's house is long, it is also difficult to install a cable therebetween. In such a place where it is difficult to install a cable, it is proposed to transmit video signals from a cable branch point to a subscriber's house on radio waves.
It is however difficult to transmit the video signals having all the several ten channels, which are transmitted on cables at frequencies of 70 MHz to 350 MHz (recently, 750 MHz), at radio frequencies because the same bandwidth as that of the cable is required. In broadband transmission, a modulation distortion generates in a nonlinear circuit such as a transmission amplifier, and leaks to an adjacent channel, in other words, adjacent channel interference is caused, so that an amplifier and a frequency converter having a high output are required, which leads to a large-scale apparatus and, hence, a rise in cost.
To avoid this problem, a cable television network (CATV network) and a radio apparatus for transmitting video signals having a narrow bandwidth from a cable branch point to a subscriber's house is demanded.